18:00 30 November 2005
NewScientist.com news service
Fred Pearce
The ocean current that gives western Europe its relatively balmy climate
is stuttering, raising fears that it might fail entirely and plunge the
continent into a mini ice age.

The dramatic finding comes from a study of ocean circulation in the
North Atlantic, which found a 30% reduction in the warm currents that carry
water north from the Gulf Stream.

The slow-down, which has long been predicted as a possible consequence
of global warming, will give renewed urgency to intergovernmental talks
in Montreal, Canada, this week on a successor to the Kyoto Protocol.

Harry Bryden at the Southampton Oceanography Centre in the UK, whose
group carried out the analysis, says he is not yet sure if the change is
temporary or signals a long-term trend. "We don’t want to say the circulation
will shut down," he told New Scientist. "But we are nervous about our findings.
They have come as quite a surprise."

No one-off
The North Atlantic is dominated by the Gulf Stream – currents that
bring warm water north from the tropics. At around 40° north – the
latitude of Portugal and New York – the current divides. Some water heads
southwards in a surface current known as the subtropical gyre, while the
rest continues north, leading to warming winds that raise European temperatures
by 5°C to 10°C.

But when Bryden’s team measured north-south heat flow last year, using
a set of instruments strung across the Atlantic from the Canary Islands
to the Bahamas, they found that the division of the waters appeared to
have changed since previous surveys in 1957, 1981 and 1992. From the amount
of water in the subtropical gyre and the flow southwards at depth, they
calculate that the quantity of warm water flowing north had fallen by around
30%.

When Bryden added previously unanalysed data – collected in the same
region by the US government’s National Oceanic and Atmospheric Administration
– he found a similar pattern. This suggests that his 2004 measurements
are not a one-off, and that most of the slow-down happened between 1992
and 1998.

The changes are too big to be explained by chance, co-author Stuart
Cunningham told New Scientist from a research ship off the Canary Islands,
where he is collecting more data. "We think the findings are robust."

Hot and cold
But Richard Wood, chief oceanographer at the UK Met Office’s Hadley
Centre for climate research in Exeter, says the Southampton team's findings
leave a lot unexplained. The changes are so big they should have cut oceanic
heating of Europe by about one-fifth – enough to cool the British Isles
by 1°C and Scandinavia by 2°C. "We haven’t seen it yet," he points
out.

Though unseasonably cold weather last month briefly blanketed parts
of the UK in snow, average European temperatures have been rising, Wood
says. Measurements of surface temperatures in the North Atlantic indicate
a strong warming trend during the 1990s, which seems now to have halted.

Bryden speculates that the warming may have been part of a global temperature
increase brought about by man-made greenhouse warming, and that this is
now being counteracted by a decrease in the northward flow of warm water.

After warming Europe, this flow comes to a halt in the waters off Greenland,
sinks to the ocean floor and returns south. The water arriving from the
south is already more saline and so more dense than Arctic seas, and is
made more so as ice forms.

Predicted shutdown
But Bryden’s study has revealed that while one area of sinking water,
on the Canadian side of Greenland, still seems to be functioning as normal,
a second area on the European side has partially shut down and is sending
only half as much deep water south as before. The two southward flows can
be distinguished because they travel at different depths.

Nobody is clear on what has gone wrong. Suggestions for blame include
the melting of sea ice or increased flow from Siberian rivers into the
Arctic. Both would load fresh water into the surface ocean, making it less
dense and so preventing it from sinking, which in turn would slow the flow
of tropical water from the south. And either could be triggered by man-made
climate change. Some climate models predict that global warming could lead
to such a shutdown later this century.

The last shutdown, which prompted a temperature drop of 5°C to 10°C
in western Europe, was probably at the end of the last ice age, 12,000
years ago. There may also have been a slowing of Atlantic circulation during
the Little Ice Age, which lasted sporadically from 1300 to about 1850 and
created temperatures low enough to freeze the River Thames in London.

· Slowing of current by a third in 12 years could bring more
extreme weather
· Temperatures in Britain likely to drop by one degree in next
decade

Ian Sample, science correspondent
Thursday December 1, 2005

Guardian

The powerful ocean current that bathes Britain and northern Europe in
warm waters from the tropics has weakened dramatically in recent years,
a consequence of global warming that could trigger more severe winters
and cooler summers across the region, scientists warn today.
Researchers on a scientific expedition in the Atlantic Ocean measured
the strength of the current between Africa and the east coast of America
and found that the circulation has slowed by 30% since a previous expedition
12 years ago.

The current, which drives the Gulf Stream, delivers the equivalent of
1m power stations-worth of energy to northern Europe, propping up temperatures
by 10C in some regions. The researchers found that the circulation has
weakened by 6m tonnes of water a second. Previous expeditions to check
the current flow in 1957, 1981 and 1992 found only minor changes in its
strength, although a slowing was picked up in a further expedition in 1998.
The decline prompted the scientists to set up a £4.8m network of
moored instruments in the Atlantic to monitor changes in the current continuously.

The network should also answer the pressing question of whether the
significant weakening of the current is a short-term variation, or part
of a more devastating long-term slowing of the flow.

If the current remains as weak as it is, temperatures in Britain are
likely to drop by an average of 1C in the next decade, according to Harry
Bryden at the National Oceanography Centre in Southampton who led the study.
"Models show that if it shuts down completely, 20 years later, the temperature
is 4C to 6C degrees cooler over the UK and north-western Europe," Dr Bryden
said.

Although climate records suggest that the current has ground to a halt
in the distant past, the prospect of it shutting down entirely within the
century are extremely low, according to climate modellers.

The current is essentially a huge oceanic conveyor belt that transports
heat from equatorial regions towards the Arctic circle. Warm surface water
coming up from the tropics gives off heat as it moves north until eventually,
it cools so much in northern waters that it sinks and circulates back to
the south. There it warms again, rises and heads back north. The constant
sinking in the north and rising in the south drives the conveyor.

Global warming weakens the circulation because increased meltwater from
Greenland and the Arctic icesheets along with greater river run-off from
Russia pour into the northern Atlantic and make it less saline which in
turn makes it harder for the cooler water to sink, in effect slowing down
the engine that drives the current.

The researchers measured the strength of the current at a latitude of
25 degrees N and found that the volume of cold, deep water returning south
had dropped by 30%. At the same time, they measured a 30% increase in the
amount of surface water peeling off early from the main northward current,
suggesting far less was continuing up to Britain and the rest of Europe.
The report appears in the journal Nature today.

Disruption of the conveyor-belt current was the basis of the film The
Day After Tomorrow, which depicted a world thrown into chaos by a sudden
and dramatic drop in temperatures. That scenario was dismissed by researchers
as fantasy, because climate models suggest that the current is unlikely
to slow so suddenly.

Marec Srokosz of the National Oceanographic Centre said: "The most realistic
part of the film is where the climatologists are talking to the politicians
and the politicians are saying 'we can't do anything about it'."

Chris West, director of the UK climate impacts programme at Oxford University's
centre for the environment, said: "The only way computer models have managed
to simulate an entire shutdown of the current is to magic into existence
millions of tonnes of fresh water and dump it in the Atlantic. It's not
clear where that water could ever come from, even taking into account increased
Greenland melting."

Uncertainties in climate change models mean that the overall impact
on Britain of a slowing down in the current are hard to pin down. "We know
that if the current slows down, it will lead to a drop in temperatures
in Britain and northern Europe of a few degrees, but the effect isn't even
over the seasons. Most of the cooling would be in the winter, so the biggest
impact would be much colder winters," said Tim Osborn, of the University
of East Anglia climatic research unit.

The final impact of any cooling effect will depend on whether it outweighs
the global warming that, paradoxically, is driving it. According to climate
modellers, the drop in temperature caused by a slowing of the Atlantic
current will, in the long term, be swamped by a more general warming of
the atmosphere.

"If this was happening in the absence of generally increasing temperatures,
I would be concerned," said Dr Smith. Any cooling driven by a weakening
of the Atlantic current would probably only slow warming rather than cancel
it out all together. Even if a slowdown in the current put the brakes on
warming over Britain and parts of Europe, the impact would be felt more
extremely elsewhere, he said.